A daemon god appears on your world, unleashes his armies, and begins terraforming your entire planet into a Giger-esque hellscape lorded over by hundreds of his daemonic demigod proxies. He and they have no known weaknesses, but could possibly be defeated one by one by a party of the world's most powerful gish. It is also rumored he has a fondness for dragons and beautiful, epic blackmetal music and will grant an audience to anyone willing to provide him with either. Little else is known about him save that he was once human and betrayed and murdered by the woman who was once his empress during his mortal life and that he brutally murdered her and devoured her soul after he was reincarnated as a daemon.

If you don't stop him somehow, your entire galaxy is fucked, because your world is its core and the origin source of all love and life in your galaxy.

What do?